Upgrading to Skip 1.9 and the Android Gradle Plugin 9.2

[marcprux]

Skip 1.9.1 upgrades its default version of the Android Gradle Plugin (AGP) from 8 to 9. This upgrade is necessary to support building apps using Java 26, which will break due to https://issuetracker.google.com/issues/486844145 with the error:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileDebugJavaWithJavac' (registered by plugin 'com.android.internal.application').
> Could not resolve all files for configuration ':app:androidJdkImage'.
   > Failed to transform core-for-system-modules.jar to match attributes {artifactType=_internal_android_jdk_image, org.gradle.libraryelements=jar, org.gradle.usage=java-runtime}.
      > Execution failed for JdkImageTransform: /Users/marc/Library/Android/sdk/platforms/android-36/core-for-system-modules.jar.
         > Error while executing process /opt/homebrew/Cellar/openjdk/26.0.1/libexec/openjdk.jdk/Contents/Home/bin/jlink with arguments {--module-path /Users/marc/.gradle/caches/9.5.1/transforms/5784d43920d64ce29e89c7bd366822f4/transformed/output/temp/jmod --add-modules java.base --output /Users/marc/.gradle/caches/9.5.1/transforms/5784d43920d64ce29e89c7bd366822f4/transformed/output/jdkImage --disable-plugin system-modules}

Caution

Unfortunately, the upgrade from AGP 8 to 9 is a major upgrade (see https://developer.android.com/build/releases/agp-9-0-0-release-notes) that will cause pre-existing Skip app projects to fail to build unless some changes are made. Fortunately, these issues can be easily rectified by making a couple edits to your project's Android/build.gradle.kts file.

Issue 1: Remove the libs.plugins.kotlin.android plugin

https://developer.android.com/build/migrate-to-built-in-kotlin#migration-steps-remove-kotlin-android-plugin

Android/app/build.gradle.kts:4:31 Unresolved reference 'android'.

Solution: simply remove the alias(libs.plugins.kotlin.android) line altogether.

Issue 2: Change proguard-android.txt to proguard-android-optimize.txt

* Where:
Build file 'Android/app/build.gradle.kts' line: 87

* What went wrong:
`getDefaultProguardFile('proguard-android.txt')` is no longer supported since it includes `-dontoptimize`, which prevents R8 from performing many optimizations. Instead use `getDefaultProguardFile('proguard-android-optimize.txt)`, and if needed, temporarily use `-dontoptimize` in a custom keep rule file while fixing breakages.

Solution: Change the getDefaultProguardFile("proguard-android.txt"), section of the proguardFiles() call to getDefaultProguardFile("proguard-android-optimize.txt").

When encountering these errors, clicking on the error in Xcode should jump straight to the offending lines in the build.gradle.kts file, so these fixes should be fairly quick to apply. Alternatively, running the commands skip upgrade (to ensure you have skip 1.9.1) then skip verify --fix from the root of your Skip app project should perform the necessary surgery on the files for you.

We understand that this sort of project breakage can be confusing and frustrating. We aim to minimize the disruption for existing Skip projects whenever we update dependencies, but since an app's Android/build.gradle.kts is generated once by Skip and is expected to be customized on a per-project basis, there wasn't any automated update that we could perform safely on these files.

P.S. The implementation for this migration is split across three different PRs: #697, skiptools/skipstone#253, and skiptools/skip-unit#19

[dfabulich]

FYI, if you're generating Baseline profiles for your Skip Android app, the latest stable Baseline Gradle Plugin 1.4.x is not compatible with AGP 9.2 unless you set android.newDsl=false in your Android/gradle.properties. (Alternately, you can upgrade the Baseline Gradle Plugin to the latest 1.5.0 alpha, if you're comfortable taking that risk.)

If you have an Android/baselineprofile/build.gradle.kts, you're going to need to upgrade that, too, upgrading com.android.test, removing org.jetbrains.kotlin.android, and (if you want) upgrading androidx.baselineprofile.

 plugins {
-    id("com.android.test") version "8.13.2"
-    id("org.jetbrains.kotlin.android") version "2.3.0"
-    id("androidx.baselineprofile") version "1.4.1"
+    id("com.android.test") version "9.2.0"
+    id("androidx.baselineprofile") version "1.5.0-alpha06"
 }

[IT-Guy007]

This is a great improvement, Thanks for the detailed migration guide, it helps a lot.

That said, I want to raise a versioning concern: this update introduces breaking changes that require manual edits to existing project files. By semantic versioning convention, breaking changes belong in a major version bump (2.0), not a minor one (1.9). A minor version should be safe to adopt without breaking existing projects.

I understand the AGP 9 upgrade was likely unavoidable due to the Java 26 incompatibility, but the impact on consumers is the same regardless of the underlying reason. For changes of this magnitude, bumping to 2.0 would set clearer expectations, developers and tools that respect semver would know to treat the upgrade with more caution, and it would signal that migration steps are required.

Hopefully something to consider for future releases of this scale.

[marcprux]

This is a very valid point, and thank you for raising it.

We have had a number of project upgrades in the past which, for various reasons, have required a Skip project to be updated in some manual way (e.g., when Gradle 9 was updated at https://github.com/orgs/skiptools/discussions/484 and when Xcode 16.3 was updated at https://github.com/orgs/skiptools/discussions/386). Skip relies on a wide variety of tools, and our policy thus far has been that we want to work with the latest/greatest releases of our dependencies, both the underlying tools (Swift, Java, Kotlin, Gradle, AGP, Xcode, Android Studio, etc.), as well as the build and runtime dependencies (like the compose and androidx packages).

We don't have a written policy on this yet, but the general attitude is that if we were to force a major version bump for every project change that was required to keep up with tooling updates, then we would be bumping the major version multiple times per year. And if people did not manually update their projects to follow the updated major versions, then their projects would still be broken when Skip becomes no longer compatible with an underlying dependency that might be out of their control (e.g., we get the openjdk and gradle dependencies from Homebrew, which doesn't consult with us when they bump versions; and of course we have no control over when Xcode releases happen, which also sometimes break Skip). Unless we were to mandate some sort of hermetically sealed environment with complete dependency control (like a Docker container), things can break out from under us at any point; a major version bump, OTOH, needs to be something that is carefully planned ahead of time. Ultimately, temporary breakage of existing projects with an informative message and release notes and tooling to help upgrade seems to us to be the "least worst" option.

Happy to discuss this further, though. FWIW, Skip isn't the only app development tool struggling with this issue: Flutter (flutter/flutter#181557) and React Native (facebook/react-native#56222 (comment)) and KMP (https://blog.jetbrains.com/kotlin/2026/05/new-kmp-default-structure/) are all having to do major restructuring to support AGP 9. See also: AGP 9.0 is Out, and Its a Disaster. Hopefully future releases of AGP will have better consideration for backwards compatibility.